Rat albumin is labeled in vivo with tritium labeled amino acids. The albumin is purified and fragmented into peptides of 6 to 30 amino acid residues by a succession of treatments such as CNBr, trypsin and chymotrypsin. The cleavage methods and peptide separation techniques are chosen so as not to differentiate between similar peptides differing by the substitution of one amino acid for another. The purified peptides are then sequenced by a modified Edman degradation to determine whether the radioactive amino acid is located only in the "correct" position. Previous experiments have shown that (3H)valine substitutes for threonine and isoleucine to the extent of about .03%. We expect to prepare in vivo labeled rat albumin from very young, young, and old rats in order to determine whether the misincorporation of a particular amino acid occurs more often in particular sites and in proportion to age.